Emissions from oil and natural gas operations north of Denver could add to ozone pollution in that region, according to a new study by researchers at the University of Colorado Boulder’s Cooperative Institute for Research in Environmental Sciences (CIRES).

“At our test site in Weld County, we found that oil and natural gas operations are the dominant wintertime source of certain gases, called volatile organic compounds (VOCs), that act as precursors—‘starting ingredients’—for ozone pollution,” said lead author Jessica Gilman, a CIRES research chemist working at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory.

Gilman’s team found high levels of these VOCs, such as propane, in that area.

“Average levels of propane were higher than the range of values reported for 28 U.S. cities,” Gilman said. “For example, they were four to nine times higher than in Houston, Texas, and Pasadena, California.”

The researchers originally went to the test site, the Boulder Atmospheric Observatory tower about 2.5 miles east of downtown Erie, in winter 2011 to study nighttime air chemistry. “The high concentrations of several of the VOCs surprised us,” Gilman said.

To discover the source of the VOCs, Gilman’s team analyzed more than 550 air samples and determined that oil and natural gas activities were the primary source of those compounds and accounted for 55 percent of the hydrocarbons that contribute to ozone formation in this area.

At the time, Weld County had more than 15,000 active oil and gas wells; it currently has about 19,000. The study was published online in the journal Environmental Science and Technology.

A component of raw natural gas, VOCs such as propane and ethane can leak during extraction, like bubbles escaping from a soda can. VOCs can then react in the air to form lung-damaging ozone pollution, a chief component of smog.

Since cars, vegetation, livestock, and other sources also emit VOCs, however, it wasn’t initially clear how much of a role oil and gas wells played in the elevated VOC levels.

“When our first measurements came out, people would ask, ‘How do you know the high level of pollutants is from natural gas? How do you know it’s not from car exhaust, or cattle farms, or people’s propane grills?’” Gilman said. “But we discovered that emissions from oil and natural gas activities have a unique ‘chemical signature’ that’s very different from emissions from other sources, and it definitively identifies oil and gas wells as the major source of the high levels of VOCs like ethane and propane.”

The researchers uncovered that signature by analyzing the chemical makeup of all the air samples—characterizing 53 different types of VOCs and comparing the results to the composition of raw natural gas.

“Each source has its own specific composition—cars look like one thing, trees like another, and so on,” Gilman said. “Just like your nose knows what a flower smells like, or coffee, or a farm, our instruments can ‘smell and identify’ oil and natural gas emissions. The signature is a very clear, robust marker.”

Once in the air, the VOC emissions can react with sunlight and nitrogen gases to form ground-level ozone pollution. Like wood fuels a campfire, Gilman said, VOCs can fuel the production of ozone pollution.

“What this study tells us is how much wood we have piled up there,” she said. “So in the wintertime at the BAO tower site, oil and natural gas operations are the dominant source of ozone precursors.”

The oil and gas footprint extends beyond Weld County, though. When the researchers took measurements near Fort Collins and in Boulder, north and west of the BAO tower respectively, they also detected emissions attributed to oil and natural gas there.

“Propane and ethane are fairly long-lived in the atmosphere, so they travel far,” Gilman said. “No matter where you are in the Front Range, you can still see the signature of VOC emissions from oil and natural gas operations.”

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“When our first measurements came out, people would ask, ‘How do you know the high level of pollutants is from natural gas? How do you know it’s not from car exhaust, or cattle farms, or people’s propane grills?’” Jessica Gilman said. “But we discovered that emissions from oil and natural gas activities have a unique ‘chemical signature’ that’s very different from emissions from other sources, and it definitively identifies oil and gas wells as the major source of the high levels of VOCs like ethane and propane.”